Project Summary/Abstract This research addresses word learning during storybook interactions in children with autism spectrum disorder (ASD) and fragile X syndrome (FXS). These disorders overlap in social communication impairments, but are associated with differences relevant to word learning: opposite strengths in the core components of language? form and meaning?and distinct memory and attention skills. Yet, surprisingly little is known about why vocabulary acquisition is altered in children with ASD and FXS. Without this information, the learning profiles associated with ASD and FXS (i.e., acquisition patterns, cognitive processing skills) cannot be used to support vocabulary development. The long-term goal of this research is to inform vocabulary interventions that meet the specific learning needs of children with ASD and FXS. The overall objective is to drastically increase what is known about word learning processes in children with ASD and FXS by determining how the lexical (form) and semantic (meaning) factors that shape word learning in typical development operate in ASD and FXS. Further, the learning context is a storybook narrative, which is drawn from promising evidence-based vocabulary treatments for other populations. The central hypothesis is that children with ASD and FXS will show opposite strengths in word learning accuracy for novel word characteristics that relate to form (ASD) and meaning (FXS), with individual differences explained by memory and attention skills. The proposed research extends a contemporary theory of word learning to ASD and FXS to explain how effects of word characteristics on learning relate to cognitive processing and vocabulary outcomes, connecting input characteristics to learning profiles in ASD and FXS. Aim 1 is to determine the impact of lexical and semantic features on word learning in a narrative context across children with ASD, FXS, and nonverbal developmental-level matched typically developing children. Immediately and 24 hours later, word learning accuracy will be assessed with an eye-gaze measure of novel word processing and a traditional production and recognition probe. Aim 2 is to identify how memory and attention explain variability in word learning performance and thereby, in vocabulary size. The proposed research is innovative because it tests learning in a meaningful context (storybook listening), assesses learning and retention over time (immediate and delayed), simultaneously varies two types of cues that influence learning (form and meaning), and directly tests the role of memory and attention in word learning. This work is significant because it will delineate (1) how features of input operate in lexical acquisition processes in ASD and FXS, (2) why input characteristics matter for learning, and (3) how and when individual characteristics?memory and attention?underlie those learning processes.